It has been proposed previously to provide seat-belt pretensioners which apply a tension to a seat-belt in the event that an accident occurs. Many pre-tensioners of this type are actuated by a pyrotechnic charge, which generates gas to move a piston within a cylinder. The piston applies tension to a wire which is wound round the spindle of a seat-belt retractor so that seat-belt is wound into the retractor, thus tightening the seat-belt across the lap and chest of a person wearing the seat-belt. Other pretensioners may move the entire retractor relative to its mounting, or may move an end part of the seat-belt, or a guide through which the seat-belt passes, or may move the seat-belt buckle, relative to a mounting. The action of the pretensioner is to ensure that the seat-belt can provide an optimum retaining effect, thus effectively retaining the person wearing the seat-belt in their seat during the accident situation and minimizing the risk of the person wearing the seat-belt moving from their position in their seat to impact with another part of the vehicle, such as a steering-wheel or dashboard.
U.S. Pat. No. 6,394,495 discloses a seat-belt tightener which has an electrical tightening device that can be operated in response to sensor signals generated by sensors which evaluate various parameters relating to the vehicle in which the arrangement is fitted. The electrical tightening device can generate a “soft” tightening profile with a relatively low seat-belt tension, or alternatively can generate a “hard” tightening profile with a higher pulling force, which gives a higher seat-belt tension. The soft tightening profile is chosen when the vehicle exhibits certain handling characteristics such as an excessive lateral acceleration, or excessive yaw angle speed or yaw angle acceleration. On the other hand the hard tightening profile is selected when other indicating signals, such as signals from a brake pressure meter, which determines the severity of braking, are higher than an assigned threshold value.
Thus the soft tightening profile is selected in response to signals from specific groups of sensors and the hard tightening profile is selected in response to signals from other groups of sensors.
Accordingly, there is a need to provide an improved seat-belt pretensioner arrangement.